The invention relates to linear actuators. In particular, the invention relates to an electrical actuator that provides a linear motion and produces a predetermined force at the end of the outward extension of the actuator.
Linear actuators are used to move machine parts along a straight path. For example, a linear actuator may be used to move a first plate toward a fixed second plate to produce a pressing or clamping action. It may be desirable for the actuator to advance and produce a predetermined force.
For example, fabrication of articles of manufacture from plastic films, such as bags and pouches, generally requires the welding together of two or more plastic films or sheets to form seams or closures. Plastic films may also be heat sealed to unlike materials such as paperboard or metal foils. Heat sealing or welding may be accomplished by pressing a heated die or seaming iron with the pattern of the required seams or closures against the plastic films or sheets with a suitable pressure for a suitable length of time.
A pouch making machine as disclosed in U.S. Pat. No. 5,800,325 is an example of fabrication that employs a heat sealing die. FIG. 1 is a schematic side view of an exemplary pouch making machine. The pouches fabricated by the machine of FIG. 1 start out as two webs of pouch material 11, 12. These two webs are joined by being seamed together to form a single web 26 in a heat seal station 10. A seaming iron 18 used to create the perimeter seams on the pouches may simultaneously form the seams for a number of pouches along the length of the web 26. The seaming iron 18 may be any number of pouches wide, as is economical under the circumstances.
Two webs of sheet stock 11, 12 are fed into the pouch making machine from two rolls 13 and 14. The webs 11, 12 are drawn into the machine by rollers 15, 16. One or both of these rollers are driven by a motor 17. The motion is intermittent in that the webs are drawn rapidly into the machine for a period of time, and then the motion stops for some other period of time to allow the perimeter seams of the pouches to be made by the hot seaming iron 18 being pressed against a platen 19. The seaming iron 18 is pressed against the platen 19 by one or more actuators 22 under the control of a control system 21 The control system 21 may use a signal from a sensor 20 to determine the position or speed of the web 26. A typical duration for the seaming process and subsequent cooling is about two seconds.
As the rollers 15, 16 are drawing the webs 11, 12 under the seaming iron 18, the web section 26 on which pouch seams have previously been formed is passed into an accumulator portion of the machine. The accumulator is the portion of the machine between rollers 15/16 and roller 23. A dancer roller 24 moves up and down to accommodate the web being fed in. The seamed web is then fed into subsequent processing stations such as a slitter and transverse cutter 30.
Three conditions involved in creating a seal are temperature, pressure, and time. At a constant temperature of the heat seal die, a reduction in pressure requires a corresponding increase in time to maintain the quality of the seal. Thus it is important that the actuator 22 accurately produces the correct pressure in each heat sealing cycle with a low cycle to cycle variation. The length of time the heat seal die 18 is held against the plastic films 11, 12 has to be long enough to produce a quality weld for the lowest pressure that may be applied. Excessive cycle to cycle variation in heat seal pressure requires that the sealing time be increased accordingly. This reduces the throughput of the heat sealing station 10.
Prior art heat sealing stations have used hydraulic or pneumatic actuators to apply pressure to the heat seal die. Hydraulic or pneumatic actuators produce a predetermined force that is related to the supply pressure of the actuating fluid. However, the use of hydraulic or pneumatic actuators requires that a source of pressurized actuating fluid be available. It would be desirable to provide an actuator to advance and produce a predetermined force without the need for a source of pressurized actuating fluid.